Solid state image pickup apparatus and radiation image pickup apparatus

ABSTRACT

In a solid state image pickup apparatus with a photodetecting device and one or more thin film transistors connected to the photodetecting device formed in one pixel, a part of the photodetecting device is formed over at least a part of the thin film transistor, and the thin film transistor is constructed by a source electrode, a drain electrode, a first gate electrode, and a second gate electrode arranged on the side opposite to the first gate electrode with respect to the source electrode and the drain electrode, and the first gate electrode is connected to the second gate electrode every pixel, thereby, suppressing an adverse effect of the photodetecting device on the TFT, a leakage at turn-off TFT, variation in a threshold voltage of the TFT due to an external electric field, and accurately transferring photo carrier to a signal processing circuit.

TECHNICAL FIELD

The invention relates to a solid state image pickup apparatus havingphotodetecting devices and thin film transistors (TFTs) and a radiationimage pickup apparatus. The solid state image pickup apparatus is usedin the radiation image pickup apparatus for detecting a radiation suchas X-ray, α-ray, β-ray, γ-ray, or the like in a medical image diagnosingapparatus, a non-destructive inspecting apparatus, an analyzingapparatus, or the like.

BACKGROUND ART

In recent years, the realization of a large size of a TFT matrix panelin which TFTs are formed on an insulating substrate and the realizationof a high driving speed have rapidly been being progressed. Amanufacturing technique of a liquid crystal panel using TFTs is used foran area sensor as a solid state image pickup apparatus havingphotodetecting devices each for converting visible light into anelectric signal. By arranging a converting layer for converting an X-ray(radiation) into a visible light beam onto the surface, such anapparatus is also used as a radiation image pickup apparatus. Accordingto such a panel which reads a light irradiation amount, unlike an imagedisplay apparatus such as a liquid crystal panel, it is particularlyimportant to accurately transfer charges accumulated in each pixel.However, for example, if a threshold voltage of the TFT is changed dueto an external action, an image variation occurs in a fetched image. Inthe radiation image pickup apparatus, therefore, the followingconditions are required of the photodetecting devices and the TFTs.

(1) An amount of light irradiated every device is accurately accumulatedas charges.

(2) The charges accumulated in each device are accurately transferred.

Owing to the progress of the recent technique of TFTs for liquid crystaldisplays, there has been proposed a radiation image pickup apparatusformed by combining: a sensor array constructed by photodetectingdevices using amorphous silicon (hereinbelow, abbreviated to a-Si) andswitching TFTs; and a phosphor for converting a radiation into visiblelight or the like. Digitization has also been accomplished in a medicalimage field. Since a radiation image can be momentarily read out,instantaneously displayed onto a display, and fetched as digital data bysuch a radiation image pickup apparatus, storage, modification,transfer, and the like of the data can be performed. However, forexample, according to a bottom gate type TFT, since source-drainelectrodes and a channel portion of the TFT are arranged in the upperportion, there is a feature such that it is influenced by an externalaction and the threshold voltage changes. Particularly, if thephotodetecting device is arranged, for example, in such a form as tocover the TFT in order to improve a numerical aperture, a back channeleffect is obtained for the TFT by an influence of electrons or holeswhich are generated in the photodetecting device and a phenomenon suchthat the threshold voltages of the TFTs of the pixels are differentoccurs.

Therefore, for example, in a solid state image pickup apparatus in whicha photodetecting device is arranged to an upper portion of a TFT, it isnecessary to cover a channel upper portion of the TFT with an electrode.

As a conventional example, according to a proposition of Japanese PatentApplication Laid-Open No. 6-216359 by Casio Computer Co., Ltd., a TFTdevice has a structure in which a source electrode and a drain electrodeare sandwiched by a top gate electrode and a bottom gate electrode. Inthe patent literature 1, since it has the structure in which asemiconductor layer of the TFT device is also used as a photoelectricconverting layer, it is difficult to obtain preferable values withrespect to both of characteristics such as a switching speed and thelike of the TFT and converting efficiency as a photoelectric convertingdevice and there is a relation of tradeoff between them.

DISCLOSURE OF THE INVENTION

The invention is made in consideration of the above problems and it isan object of the invention to provide a solid state image pickupapparatus of low costs and high performance which comprisesphotodetecting devices and thin film transistors and has a structure inwhich the photodetecting device covers a part or a whole surface of thethin film transistor, particularly, in order to form a large openingportion of the photodetecting device, while the stable thin filmtransistor of high performance which accurately transfers generatedcharges to a signal processing circuit is provided, and characteristicsof both of the thin film transistor and the photodetecting device can beindependently set.

A solid state image pickup apparatus of the invention is characterizedin that a photodetecting device and one or more thin film transistorsconnected to the photodetecting device are formed in one pixel, a partof the photodetecting device is formed over at least a part of the thinfilm transistor, the thin film transistor comprises a source electrode,a drain electrode, a first gate electrode, and a second gate electrodearranged on the side opposite to the first gate electrode with respectto the source electrode and the drain electrode, and the first gateelectrode is connected to the second gate electrode every pixel.

Thus, it is possible to provide the stable thin film transistor of highperformance in which an influence of the photodetecting device formedover the upper portion of the TFT is eliminated, when the TFT is turnedoff, a leakage is small, and a threshold voltage of the TFT is notchanged by an electric field from an outside but generated charges areaccurately transferred to a signal processing circuit. In addition, thesolid state image pickup apparatus of low costs and high performance inwhich the characteristics of both of the thin film transistor and thephotodetecting device can be independently set can be provided. Sincethe TFT can be controlled by the two gate electrodes, the number ofchannels of the TFT increases and it also contributes to the improvementof transfer efficiency of the charges.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a plan view of a pixel of a solid state image pickup apparatusin an embodiment 1 of the invention.

FIG. 2 is a cross sectional view taken along the line 2-2 in FIG. 1.

FIG. 3 is a plan view of a pixel of a solid state image pickup apparatusin another example of the embodiment 1 of the invention.

FIG. 4 is a cross sectional view taken along the line 4-4 in FIG. 3.

FIG. 5 is a simple equivalent circuit diagram of the solid state imagepickup apparatus and its peripheral circuit diagram in the embodiment 1of the invention.

FIG. 6 is a plan view of a pixel of a solid state image pickup apparatusin an embodiment 2 of the invention.

FIG. 7 is a cross sectional view taken along the line 7-7 in FIG. 6.

FIG. 8 is a cross sectional view of a pixel of a solid state imagepickup apparatus in an embodiment 3 of the invention.

FIG. 9 is a plan view of a pixel of a solid state image pickup apparatusin an embodiment 4 of the invention.

FIG. 10 is a cross sectional view taken along the line 10-10 in FIG. 9.

FIG. 11 is a cross sectional view of a pixel of a solid state imagepickup apparatus in an embodiment 5 of the invention.

FIG. 12 is a schematic equivalent circuit diagram in an embodiment 6 ofthe invention.

BEST MODES FOR CARRYING OUT THE INVENTION

A solid state image pickup apparatus and a radiation image pickupapparatus according to embodiments of the invention will be specificallyexplained hereinbelow.

Embodiment 1

FIGS. 1 to 5 show plan views and cross sectional views of one pixel ofan embodiment 1 of the invention.

FIG. 1 is a layout diagram showing a plane structure of a pixelincluding a pair of a photodetecting device and a TFT in the solid stateimage pickup apparatus according to the embodiment 1 of the invention.

The photodetecting device of the embodiment is a device for convertingvisible light into charges and a phosphor layer as a wavelengthconverter for converting a radiation into the visible light is arrangedin an upper portion of the device.

A TFT (thin film transistor) 102 is constructed by four electrodes: asource electrode; a drain electrode; a first gate electrode; and asecond gate electrode. A transfer wiring 104 connected to a signalprocessing circuit for reading accumulated charges is connected to asource electrode 115 a of the TFT. A gate wiring 103 connected to a gatedriver circuit for controlling ON/OFF of the TFT is connected to a firstgate electrode 111 and also connected to a second gate electrode 117 viaa through-hole 106 every pixel. Further, a photodetecting device 101 isMIS type photodetecting device constructed sequentially from the bottomby an electrode layer, an insulating layer, an intrinsic semiconductorlayer, and an n-type semiconductor layer. One of the two electrodesconstructing the photodetecting device is connected to a drain electrode115 b of the TFT and the other electrode is connected to a bias wiring105 for applying a voltage to a sensor.

As mentioned above, by sandwiching a channel portion between the sourceelectrode and the drain electrode of the TFT 102 by the first gateelectrode 111 and a second gate electrode 117, even if electrons andholes are generated in the photodetecting device 101 arranged in theupper portion of the TFT 102 and an electric potential of the electrodesconstructing the photodetecting device fluctuates, the TFT 102 existingin the lower portion is not influenced and characteristics do notfluctuate. The gate wiring 103 can be formed by a first electrode layerwhich is used in the first gate electrode 111 or can be also formed by athird electrode layer which is used in the second gate electrode 117.However, to reduce a capacitance formed in a portion between the gatewiring 103 and the transfer wiring 104, the bias wiring 105, or a lowerelectrode of the photodetecting device 101, it is desirable to form itby the first electrode layer which is used in the first gate electrode111.

In the embodiment, particularly, in the case where a material forphotoelectrically converting the visible light is used for aphotodetecting device, it is desirable that the light does not enter thegap portion between the source and the drain of the TFT. Therefore, itis desirable that a lower electrode layer which is used for the lowerelectrode of the photodetecting device which is arranged in the upperportion of the TFT and the electrode layer which is used as a secondgate electrode of the TFT is not formed by a transparent electrode layersuch as ITO or the like but a metal layer such as Al or Mo which doesnot transmit the light is used.

FIG. 2 is a cross sectional view taken along the line 2-2 in FIG. 1.

Each layer is formed on an insulating substrate (not shown). A phosphorlayer 175 is arranged in an upper portion. The TFT 102 is arranged in aright portion. The photodetecting device 101 is arranged in a leftportion so as to cover the TFT 102 in the right portion. The TFT 102 hasa structure of a bottom gate type and is constructed sequentially fromthe bottom by: the first gate electrode 111 comprising the firstelectrode layer; source-drain electrodes comprising a first insulatinglayer 112, a first intrinsic semiconductor layer 113, a first n-typesemiconductor layer 114, and a second electrode layer 115; and thesecond gate electrode 117 comprising a second insulating layer 116 and athird electrode layer.

The photodetecting device 101 is constructed sequentially from thebottom by: a fourth electrode layer 122; a fourth insulating layer 123;a second intrinsic semiconductor layer 124; and a second n-typesemiconductor layer 125. A bias wiring comprising a fifth electrodelayer 126 which has a low resistance and to which a bias can be appliedis connected to the second n-type semiconductor layer 125. Such astructure is possible in the case where a resistance of the n-typesemiconductor layer is low, for example, like a microcrystal n-typesemiconductor layer. If the resistance of the n-type semiconductor layeris high like an amorphous silicon n-type semiconductor layer, it isnecessary to form an electrode layer onto the whole upper surface of then-type semiconductor layer. It is preferable that, for example, an ITOlayer serving as a transparent electrode layer which sufficientlytransmits the visible light is used as such an electrode layer. Thedrain electrode of the TFT 102 (the left portion of the second electrodelayer 115 in FIG. 2) is connected to an electrode comprising the fourthelectrode layer 122 of the photodetecting device 101. A third insulatinglayer 121 is arranged in a lower portion. A fifth insulating layer 127is arranged in an upper portion.

In the solid state image pickup apparatus of such a structure, when thevisible light converted in the phosphor from the radiation enters thephotodetecting device 101, electrons and holes are generated in theintrinsic semiconductor layer. For example, if the second n-typesemiconductor layer 125 of the photodetecting device 101 or theelectrode layer which is arranged in its upper portion and uses, forexample, ITO has been fixed to a predetermined electric potential, apotential fluctuation is caused in the lower electrode of thephotodetecting device 101 comprising the fourth electrode layer 122.Although an image can be displayed by reading the potential fluctuationvia the TFT, if such a potential fluctuation occurs on the gap portionbetween the source and the drain of the TFT, a back channel effect ofthe TFT is caused and it becomes a factor which changes a thresholdvoltage of the TFT. Therefore, the second gate electrode 117 comprisingthe third electrode layer in FIG. 2 is arranged between the portion onthe gap between the source and the drain of the TFT and the lowerelectrode of the photodetecting device via the insulating layer andconnected to the first gate electrode 111 of the TFT comprising thefirst electrode layer, so that an influence of the photodetecting devicecan be prevented.

FIG. 3 is a layout diagram in which the characteristics have beenfurther improved from those of the plane structure of the pixel in FIG.1.

The TFT 102 is constructed by four electrodes: a source electrode; adrain electrode; a first gate electrode; and a second gate electrode.Particularly, in the photoelectric converting device comprising thephotodetecting device and the TFT, it is known that when a capacitanceof the transfer wiring 104 increases, noises increase at the time ofreading out the charges generated in the photodetecting device.Therefore, as shown in FIG. 3, a channel portion between the sourceelectrode and the drain electrode of the TFT is sandwiched by the firstgate electrode 111 and the second gate electrode 117 and since thesecond gate electrode 117 does not overlap the source electrode 115 a ofthe TFT, an influence on the TFT from the photodetecting device isprevented, a capacitance which is caused between the second gateelectrode 117 and the transfer wiring 104 is reduced, and performance ofthe solid state image pickup apparatus is maintained.

FIG. 4 is a cross sectional view taken along the line 4-4 in FIG. 3.

Each layer is formed on an insulating substrate (not shown). The secondgate electrode 117 comprising the third electrode layer is arranged onthe gap between the source and the drain of the TFT and connected to thefirst gate electrode 111 of the TFT 102 comprising the first electrodelayer, so that the influence of the photodetecting device can beprevented. By arranging the second gate electrode so as not to overlapthe source electrode 115 a connected to the transfer wiring, acapacitance which is caused between the second gate electrode 117 andthe source electrode 115 a can be suppressed. Therefore, the capacitanceof the transfer wiring 104 is minimized. It is also possible to arrangethe second gate electrode 117 onto the source electrode 115 a so thatthe first gate electrode does not overlap the source electrode 115 a. Ifa transfer ability of the TFT is sufficient, it is also possible toarrange both of the second gate electrode 117 and the first gateelectrode so as not to overlap the source electrode 115 a.

A sensor panel of the solid state image pickup apparatus and itsperipheral circuit will now be described.

FIG. 5 is a simple equivalent circuit diagram of the solid state imagepickup apparatus and its peripheral circuit diagram in an embodiment 1of the invention.

A sensor panel 181 including an equivalent circuit is arranged at thecenter and signal processing circuits 182, a gate driver circuit 183,and a refresh driver circuit 184 are arranged around the outside of thesensor panel 181. The transfer wiring 104 in the panel is processed bythe signal processing circuits 182 arranged in the upper and lowerpositions in FIG. 5. The gate wiring 103 in the panel is controlled bythe gate driver circuit 183. The bias wiring 105 in the panel iscontrolled by the refresh driver circuit 184. The refresh driver circuit104 is vertically divided into two parts and connected to the upper andlower signal processing circuits 182, respectively. The bias wiring 105is led to all pixels from the dividing position where it is verticallydivided. The bias wiring 105 can be provided in the signal processingcircuit and led. Although the gate wiring 103 is controlled by the gatedriver circuit 183 arranged in the left portion, it is possible toarrange the gate driver circuits 183 to the right and left and controlthe gate wiring 103 from both directions or it is also possible todivide the gate wiring 103 in the center portion and independentlycontrol the divided right and left gate wirings.

Embodiment 2

FIG. 6 is a layout diagram showing a plane structure of a pixelincluding a pair of a photodetecting device and a TFT of a solid stateimage pickup apparatus in an embodiment 2 of the invention.

The photodetecting device of the embodiment is a device for convertingvisible light into charges. A phosphor layer serving as a wavelengthconverter for converting a radiation into visible light is arranged inan upper portion of the device.

A TFT (thin film transistor) 102 is constructed by four electrodes: asource electrode; a drain electrode; a first gate electrode; and asecond gate electrode. The transfer wiring 104 connected to the signalprocessing circuit for reading out the accumulated charges is connectedto the source electrode 115 a of the TFT. The gate wiring 103 connectedto the gate driver circuit 183 for controlling ON/OFF of the TFT isconnected to the first gate electrode 111 of the TFT and also connectedto the second gate electrode 117 via the through-hole 106 every pixel.Further, the photodetecting device 101 is an MIS type photodetectingdevice constructed sequentially from the bottom by the electrode layer,insulating layer, intrinsic semiconductor layer, and n-typesemiconductor layer. One of the two electrodes constructing thephotodetecting device is connected to the drain electrode 115 b of theTFT and the other electrode is connected to the bias wiring 105 forapplying a voltage to the sensor.

The lower electrode of the photodetecting device 101 is not arranged onthe TFT 102 but the TFT 102 is constructed sequentially from the bottomby the insulating layer, intrinsic semiconductor layer, n-typesemiconductor layer, and electrode layer. However, if the second gateelectrode 117 does not exist, when the radiation enters, in theembodiment, the holes are accumulated into an interface of theinsulating layer and the intrinsic semiconductor layer, particularly, onthe source and drain electrodes, so that an influence is exercised onthe TFT and it becomes a factor of changing the threshold voltage.Therefore, as shown in the diagram, by sandwiching the channel portionbetween the source electrode and the drain electrode of the TFT by thefirst gate electrode 111 and the second gate electrode 117, even if theelectrons and holes are generated in the photodetecting device arrangedin the upper portion of the TFT and the electrons and holes areaccumulated in the interface of, particularly, the insulating film andthe intrinsic semiconductor layer on the source and drain electrodesconstructing the photodetecting device, the TFT existing in the lowerportion is not influenced and the characteristics do not fluctuate.

In the embodiment, particularly, in the case where a material forphotoelectrically converting the visible light is used for thephotodetecting device, it is desirable that the light does not enter thegap portion between the source and the drain of the TFT. Therefore, itis desirable that a metal layer such as Al or Mo which is not formed ina transparent electrode layer such as ITO or the like and does nottransmit the light is used for the lower electrode layer which is usedfor the lower electrode of the photodetecting device that is arranged inthe upper portion of the TFT and for the electrode layer which is usedas a second gate electrode of the TFT.

FIG. 7 is a cross sectional view taken along the line 7-7 in FIG. 6.

Each layer is formed on an insulating substrate (not shown). Althoughnot shown, a phosphor layer is arranged in the upper portion. The TFT102 is arranged in the right portion and the photodetecting device 101is arranged in the left portion so as to cover the TFT 102 in the rightportion. The TFT 102 has a structure of a bottom gate type and isconstructed sequentially from the bottom by: the first gate electrode111 comprising the first electrode layer; the source-drain electrodescomprising the first insulating layer 112, the first intrinsicsemiconductor layer 113, the first n-type semiconductor layer 114, andthe second electrode layer 115; and the second gate electrode 117comprising the second insulating layer 116 and the third electrodelayer.

The photodetecting device 101 is constructed sequentially from thebottom by: a third electrode layer 131; a third insulating layer 132; asecond intrinsic semiconductor layer 133; a second n-type semiconductorlayer 134; and a fourth electrode layer 135. A bias wiring comprising afifth electrode layer 136 which has a low resistance and to which a biascan be applied is connected to the fourth electrode layer 135. It ispreferable that, for example, an ITO layer serving as a transparentelectrode layer for sufficiently transmitting the visible light is usedas a fourth electrode layer 135. The second gate electrode 117 as athird electrode layer arranged in the upper portion of the TFT 102 isformed simultaneously with the third electrode layer 131 as the samelayer as the lower electrode layer of the photodetecting device. Thedrain electrode 115 b of the TFT is connected to the electrodecomprising the third electrode layer 131 of the photodetecting device. Afourth insulating layer 137 is, arranged in the upper portion.

In the solid state image pickup apparatus having such a construction,when the visible light converted from the radiation by the phosphorenters the photodetecting device, electrons and holes are generated inthe intrinsic semiconductor layer. If the second gate electrode does notexist here, for example, if the fourth electrode layer 135 of thephotodetecting device is fixed to a predetermined electric potential,the lower electrode of the photodetecting device comprising the thirdelectrode layer 131 causes a potential fluctuation, and at the sametime, the holes are accumulated in the interface of the third insulatinglayer 132 and the second intrinsic semiconductor layer 133 on the sourceand drain electrodes of the TFT. The back channel effect of the TFT iscaused by such an influence and the threshold voltage of the TFT ischanged. Therefore, the second gate electrode 117 comprising the thirdelectrode layer in FIG. 7 is arranged between the portion on the gapbetween the source and the drain of the TFT and the photodetectingdevice via the insulating film and connected to the first gate electrode111 of the TFT comprising the first electrode layer, so that theinfluence of the photodetecting device can be prevented.

Embodiment 3

FIG. 8 is a cross sectional view of a pixel including a pair of aphotodetecting device and a TFT of a solid state image pickup apparatusin an embodiment 3 of the invention.

A layout diagram showing a plane structure is similar to that in theembodiment 1. The photodetecting device in the embodiment is a devicefor converting the visible light into charges and each layer is formedon an insulating substrate (not shown). A phosphor layer (not shown) forconverting the radiation into the visible light is arranged in an upperportion.

The TFT 102 is arranged in the right portion and the photodetectingdevice 101 is arranged in the left portion so as to cover the TFT 102 inthe right portion. The TFT 102 has a structure of a bottom gate type andis constructed sequentially from the bottom by: the first gate electrode111 comprising the first electrode layer; the source-drain electrodescomprising the first insulating layer 112, the first intrinsicsemiconductor layer 113, the first n-type semiconductor layer 114, andthe second electrode layer 115; and the second gate electrode 117comprising the second insulating layer 116 and the third electrodelayer.

The photodetecting device 101 is a PIN-type photodetecting deviceconstructed sequentially from the bottom by: a fourth electrode layer142; a second n-type semiconductor layer 143; a second intrinsicsemiconductor layer 144; a p-type semiconductor layer 145; and a fifthelectrode layer 146. A bias wiring comprising a sixth electrode layer147 which has a low resistance and to which a bias can be applied isconnected to the fifth electrode layer 146. It is preferable that, forexample, an ITO layer serving as a transparent electrode layer forsufficiently transmitting the visible light is used as a fifth electrodelayer 146. The drain electrode of the TFT is connected to the electrodecomprising the fourth electrode layer 142 of the photodetecting device.The third insulating electrode 132 is arranged in the lower portion anda fourth insulating layer 148 is arranged in the upper portion.

In the solid state image pickup apparatus having such a construction,when the visible light converted from the radiation by the phosphorenters the photodetecting device 101, electrons and holes are generatedin the intrinsic semiconductor layer. If the second gate electrode 117does not exist here, for example, if the fifth electrode layer 146 ofthe photodetecting device 101 is fixed to a predetermined electricpotential, the lower electrode of the photodetecting device comprisingthe fourth electrode layer 142 causes a potential fluctuation. The backchannel effect of the TFT is caused by such an influence and thethreshold voltage of the TFT is changed. Therefore, the second gateelectrode 117 comprising the third electrode layer in FIG. 8 is arrangedbetween the portion on the gap between the source and the drain of theTFT and the lower electrode of the photodetecting device 101 via theinsulating film and connected to the first gate electrode 111 of the TFTcomprising the first electrode layer, so that the influence of thephotodetecting device 101 can be prevented.

In the embodiment, the photodetecting device can be also replaced with adirect converting material for directly and photoelectrically convertingthe radiation. In this case, there is no need to arrange the phosphorlayer (not shown) which is arranged in the upper portion disclosedabove.

Embodiment 4

FIG. 9 is a layout diagram showing a plane structure of a pixelcomprising one photodetecting device and two TFTs constructing a pair ina solid state image pickup apparatus in an embodiment 4 of theinvention.

The photodetecting device in the embodiment is a device for convertingthe visible light into charges and a phosphor layer for converting theradiation into the visible light is arranged in an upper portion of thedevice.

Each of TFTs 107 and 108 is constructed by four electrodes: a sourceelectrode; a drain electrode; a first gate electrode; and a second gateelectrode. The TFT 107 in an upper right position is arranged totransfer the charges accumulated in the photodetecting device 101 to thesignal processing circuit. The TFT 108 in a lower left position isarranged to transfer the charges accumulated in the photodetectingdevice 101 and, thereafter, reset the photodetecting device 101. Thetransfer wiring 104 connected to the source electrode 115 a is led tothe signal processing circuit for reading out the accumulated charges.The gate wiring 103 connected to the gate driver circuit for controllingON/OFF of the TFT is connected to the first gate electrode 111 of theTFT and also connected to the second gate electrode 117 via thethrough-hole 106 every pixel.

Further, the photodetecting device 101 is an MIS type photodetectingdevice constructed sequentially from the bottom by an electrode layer,an insulating layer, an intrinsic semiconductor layer, and an n-typesemiconductor layer. One of the two electrodes constructing thephotodetecting device is connected to drain electrodes of the two TFTsand the other electrode is connected to the bias wiring 105 for applyinga voltage to the sensor. If the second gate electrode 117 does not existhere, when the radiation enters, the lower electrode of thephotodetecting device causes a potential fluctuation, so that the TFT isinfluenced and it becomes a factor which changes the threshold voltage.Therefore, as shown in the diagram, in the TFTs in both of the upperright portion and the lower left portion, by sandwiching the channelportion between the source electrode and the drain electrode by thefirst gate electrode 111 and the second gate electrode 117, even if thevisible light converted from the radiation is irradiated to thephotodetecting device arranged in the upper portion of the TFT, the TFTexisting in the lower portion is not influenced and the characteristicsdo not fluctuate.

In the embodiment, particularly, in the case where a material forphotoelectrically converting the visible light is used as aphotodetecting device, it is desirable that the light does not enter thegap portion between the source and the drain of the TFT. Therefore, alower electrode layer which is used for the lower electrode of thephotodetecting device which is arranged in the upper portion of the TFTor the electrode layer which is used as a second gate electrode of theTFT is not formed by a transparent electrode layer such as ITO or thelike but it is desirable to use a metal layer such as Al or Mo whichdoes not transmit the light.

FIG. 10 is a cross sectional view taken along the line 10-10 in FIG. 9.

Each layer is formed on an insulating substrate (not shown). Thephotodetecting device 101 is arranged in the upper portion so as tocover the right and left TFTs. The TFT 107 for transfer is arranged inthe right portion and the TFT 108 for resetting is arranged in the leftportion. Both of the TFTs have a structure of a bottom gate type andeach TFT is constructed sequentially from the bottom by: the first gateelectrode 111 comprising the first electrode layer; the source-drainelectrodes comprising the first insulating layer 112, the firstintrinsic semiconductor layer 113, the first n-type semiconductor layer114, and the second electrode layer 115; and the second gate electrode117 comprising the second insulating layer 116 and the third electrodelayer.

The photodetecting device is constructed sequentially from the bottomby: the fourth electrode layer 122; the fourth insulating layer 123; thesecond intrinsic semiconductor layer 124; the 2nd n-type semiconductorlayer 125; and the fifth electrode layer 126. A bias wiring comprising asixth electrode layer 128 which has a low resistance and to which a biascan be applied is connected to the fifth electrode layer 126. It ispreferable that, for example, an ITO layer serving as a transparentelectrode layer for sufficiently transmitting the visible light is usedas a fifth electrode layer 126. The drain electrode of the TFT isconnected to the electrode comprising the fourth electrode layer 122 ofthe photodetecting device. The third insulating layer 121 is arranged inthe lower portion and the fifth insulating layer 127 is arranged in theupper portion.

In the solid state image pickup apparatus having such a construction,when the visible light converted from the radiation by the phosphorenters the photodetecting device, electrons and holes are generated inthe intrinsic semiconductor layer. If the second gate electrode 117 doesnot exist here, for example, if the second n-type semiconductor layer ofthe photodetecting device is fixed to a predetermined electricpotential, the lower electrode of the photodetecting device comprisingthe fourth electrode layer causes a potential fluctuation, so that theback channel effect of the TFT is caused by such an influence and thethreshold voltage of the TFT is changed. Therefore, the second gateelectrode 117 comprising the third electrode layer in FIG. 10 isarranged between the portion on the gap between the source and the drainof each of the right and left TFTs and the lower electrode of thephotodetecting device via the insulating film and connected to the firstgate electrode 111 of the TFT comprising the first electrode layer, sothat the influence of the photodetecting device can be prevented.

In the embodiment, the photodetecting device can be replaced with aPIN-type photodetecting device or can be also replaced with a directconverting material for directly and photoelectrically converting theradiation. In the case of using the direct converting material, there isno need to arrange the phosphor layer (not shown) which is arranged inthe upper portion disclosed above.

Embodiment 5

FIG. 11 is a cross sectional view of a pixel including a pair of aphotodetecting device and a TFT of a solid state image pickup apparatusin an embodiment 5 of the invention.

A layout diagram showing a plane structure is similar to that in theembodiment 1. The photodetecting device in the embodiment is a devicefor converting the visible light into charges and each layer is formedon an insulating substrate (not shown). A phosphor layer (not shown) forconverting the radiation into the visible light is arranged in an upperportion.

The TFT 102 is arranged in a right portion. The photodetecting device101 is arranged in a left portion so as to cover the TFT 102 in theright portion. The TFT 102 has a structure of a top gate type and isconstructed sequentially from the bottom by: a first gate electrode 151comprising the first electrode layer; source-drain electrodes comprisinga second electrode layer 153; a second gate electrode 157 comprising afirst n-type semiconductor layer 154, a first intrinsic semiconductorlayer 155, a second insulating layer 156, and a third electrode layer;and a third insulating layer 161. A first insulating layer 152 is formedin the lower portion of the source and drain electrodes so that theinsulating substrate and the channel portion are not come into directcontact with each other. The first gate electrode 151 as a firstelectrode layer is arranged between the first insulating layer 152 andthe insulating substrate. The device is not influenced by the microionsincluded in the insulating substrate.

The photodetecting device 101 is constructed sequentially from thebottom by: a fourth electrode layer 162; a fourth insulating layer 163;a second intrinsic semiconductor layer 164; a second n-typesemiconductor layer 165; and a fifth electrode layer 166. A bias wiringcomprising a sixth electrode layer which has a low resistance and towhich a bias can be applied is connected to the fifth electrode layer166. It is preferable that, for example, an ITO layer serving as atransparent electrode layer which sufficiently transmits the visiblelight is used as a fifth electrode layer 166. The drain electrode of theTFT 102 is connected to an electrode comprising the fourth electrodelayer 162 of the photodetecting device 101. A third insulating layer 161is arranged in a lower portion. A fifth insulating layer 167 is arrangedin an upper portion.

In the solid state image pickup apparatus of such a structure, when thevisible light converted in the phosphor from the radiation enters thephotodetecting device 101, electrons and holes are generated in theintrinsic semiconductor layer. If the second gate electrode 157 does notexist, for example, if the fifth electrode layer 166 of thephotodetecting device has been fixed to a predetermined electricpotential, a potential fluctuation is caused in the lower electrode ofthe photodetecting device comprising the fourth electrode layer 162. Theback channel effect of the TFT is caused by such an influence and athreshold voltage of the TFT is changed. Therefore, the second gateelectrode 157 comprising the third electrode layer in FIG. 11 isarranged between the portion on the gap between the source and the drainof the TFT and the lower electrode of the photodetecting device via theinsulating layer and connected to the first gate electrode 151 of theTFT comprising the first electrode layer, thereby enabling an influenceof the photodetecting device to be prevented.

In the embodiment, the photodetecting device can be replaced with adirect converting material for directly and photoelectrically convertingthe radiation. In this case, there is no need to arrange the phosphorlayer (not shown) which is arranged in the upper portion disclosedabove.

Embodiment 6

FIG. 12 is a schematic equivalent circuit diagram in an embodiment 6 ofthe invention.

Explanation will now be made with respect to an example in which a TFTfor switching, an MIS type photodetecting device, and a TFT for readingcomprising a gate which receives charges generated in the MIS typephotodetecting device and source and drain electrodes for reading out asignal according to an amount of charges are formed by using five layersof flat films and metal films.

In FIG. 12, a common drive wiring 201 of a switching TFT 001 isconnected to a gate driver 002 for controlling ON/OFF of the switchingTFT 001. Further, a source or drain electrode of the TFT 001 isconnected to a common signal wiring 202 via a reading TFT 014. Thesignal wiring 202 is connected to an amplifier IC 003. One of electrodesof a photodetecting device 004 is connected to a common electrode driver(not shown) and the other electrode is connected to a control electrode(gate electrode) of the reading TFT 014. A control electrode of aresetting TFT 015 is connected to a drive wiring 203, either a sourceelectrode or a drain electrode is connected to the control electrode ofthe reading TFT 014, and the other electrode is connected to a resettingwiring 205.

The radiation which entered a specimen is attenuated by the specimen,transmits, and is converted into visible light by the phosphor layer.The visible light enters the photodetecting device 004 and convertedinto charges. The charges cause a potential fluctuation according to alight irradiation amount in the control electrode of the reading TFT014. An amount of current flowing in the reading TFT 014 is changed dueto the potential fluctuation and can be read out via the signal wiring202. The switching TFT 001 is used as a switching transistor forapplying a voltage across a source and a drain of the reading TFT 014.The signal is transferred to the signal wiring 202 and read out to theoutside by the amplifier IC 003. After the signal was read out, bydriving the resetting TFT 015 and applying a voltage to the electrodesof the photodetecting device 004 connected to the resetting TFT 015 viathe reset wiring 205, the charges accumulated in the photodetectingdevice can be removed.

In such a solid state image pickup apparatus of the source followertype, it is necessary to arrange a plurality of TFTs (for example, thereading TFT and the resetting TFT in the embodiment) besides the TFTincluding the first intrinsic semiconductor layer and the photodetectingdevice including the second intrinsic semiconductor layer. A capacitorcan be arranged in the circuit. In such a case, the number of variationsof design can be increased by forming them by the five layers of flatfilms and metal films as mentioned above. For example, it is possible touse a free layout such that the switching TFT, resetting TFT, andcapacitor are formed in the lower layer and the photodetecting deviceand the reading TFT are formed in the upper layer, or the like.

However, when the visible light enters the photodetecting devicearranged in the upper portion or when the TFT likewise arranged in theupper portion is made operative, the back channel effect is caused inthe TFT arranged in the lower portion. Such a phenomenon typicallyoccurs, particularly, in the bottom gate type TFT. If the photodetectingdevice or the TFT is arranged in the portion on the channel portionbetween the source and the drain, the threshold voltage of the TFT isnot stabilized but a leakage between the source and the drain is caused.Therefore, by arranging the first gate electrode and the second gateelectrode so as to sandwich the source and the drain and connecting anddriving them every pixel, the TFT can be protected against the ambientexternal action.

According to the embodiments, the stable TFT can be provided in thesolid state image pickup apparatus comprising, at least: the TFTs (forexample, the switching TFT and the resetting TFT) including the firstintrinsic semiconductor layer; and the photodetecting device includingthe second intrinsic semiconductor layer arranged on the upper surfaceof them and the electrode.

Although the structure in which the first gate electrode and the secondgate electrode are connected, connected to the same gate driver, anddriven has been shown in each of the above embodiments, it is alsopossible to connect the first gate electrode and the second gateelectrode to different drivers and, for example, change a value of avoltage which is applied.

Embodiment 7

A manufacturing method of a solid state image pickup apparatus of theinvention will be described in the embodiment 7.

The manufacturing method of the solid state image pickup apparatuscharacterized in that the apparatus comprises a substrate, aphotodetecting device arranged on the substrate, and a plurality of thinfilm transistors connected to the photodetecting device, a part of thephotodetecting device is arranged so as to overlap at least a part ofthe thin film transistor, and the thin film transistor comprises asource electrode, a drain electrode, a first gate electrode, and asecond gate electrode arranged on the side opposite to the first gateelectrode as a bottom electrode with respect to the source electrode andthe drain electrode comprises the following steps (1) to (7).

(1) Step of forming a sensor electrode of the photodetecting device anda conductive film for the gate electrodes of the thin film transistoronto the substrate.

(2) Step of forming the sensor electrode of the photodetecting deviceand the first gate electrode of the thin film transistor by patterningthe conductive film.

(3) Step of forming a common electrode of the photodetecting device anda conductive film for the source electrode and the drain electrode ofthe thin film transistor into an upper portion of the substrate.

(4) Step of forming the common electrode by patterning the conductivefilm.

(5) Step of forming the source electrode and the drain electrode of thethin film transistor by further patterning the conductive film.

(6) Step of forming a conductive film for the second gate electrode intoa further upper portion of an insulating film on the conductive film.

(7) Step of forming the second gate electrode by patterning theconductive film.

The embodiments of the invention have been described above. Preferredembodiments of the invention will be mentioned as follows.

Embodiment 1

A solid state image pickup apparatus characterized in that aphotodetecting device and one or more thin film transistors connected tothe photodetecting device are formed in one pixel, a part of thephotodetecting device is formed over at least a part of the thin filmtransistor, and the thin film transistor comprises a source electrode, adrain electrode, a first gate electrode, and a second gate electrodearranged on the side opposite to the first gate electrode with respectto the source electrode and the drain electrode.

Embodiment 2

A solid state image pickup apparatus according to Embodiment 1,characterized in that the thin film transistor is a double gate typethin film transistor comprising at least the first gate electrode, aninsulating layer, a semiconductor layer, a semiconductor layer having animpurity doped, the source and drain electrodes, an insulating layer,and the second gate electrode which are sequentially formed onto aninsulating substrate.

Embodiment 3

A solid state image pickup apparatus according to Embodiment 1 or 2,characterized in that the second gate electrode covers at least a partof a gap portion between the source electrode and the drain electrode.

Embodiment 4

A solid state image pickup apparatus according to any one of Embodiments1 to 3, characterized in that either the source electrode or the drainelectrode is connected to a transfer wiring connected to a signalprocessing circuit, and the second gate electrode does nottwo-dimensionally overlap either the source electrode or the drainelectrode connected to the transfer wiring.

Embodiment 5

A solid state image pickup apparatus according to any one of Embodiments1 to 4, characterized in that the second gate electrode and the firstgate electrode are connected to one gate driver circuit by a gate wiringand controlled by the gate driver circuit.

Embodiment 6

A solid state image pickup apparatus according to any one of Embodiments1 to 5, characterized in that the second gate electrode is formed as afilm simultaneously with an electrode material constructing thephotodetecting device.

Embodiment 7

A solid state image pickup apparatus according to any one of Embodiments1 to 6, characterized in that the photodetecting device is constructedby at least an insulating layer, a semiconductor layer, and asemiconductor layer having a impurity doped.

Embodiment 8

A solid state image pickup apparatus according to any one of Embodiments1 to 6, characterized in that the photodetecting device is constructedby at least a first semiconductor layer having a impurity doped, asemiconductor layer, and a second semiconductor layer having a impuritydoped of a conductivity type opposite to that of the first semiconductorlayer having a impurity doped.

Embodiment 9

A radiation image pickup apparatus characterized in that thephotodetecting device of the solid state image pickup apparatusaccording to any one of Embodiments 1 to 6 is a radiation detectingdevice for directly and photoelectrically converting a radiation.

Embodiment 10

A radiation image pickup apparatus characterized in that a wavelengthconverter is arranged onto the photodetecting device of the solid stateimage pickup apparatus according to any one of Embodiments 1 to 8.

Embodiment 11

A manufacturing method of a solid state image pickup apparatus

which has a substrate, a photodetecting device arranged on thesubstrate, and a plurality of thin film transistors connected to thephotodetecting device, and

in which a part of the photodetecting device is formed over at least apart of the thin film transistor, and

the thin film transistor comprises a source electrode, a drainelectrode, a first gate electrode, and a second gate electrode arrangedon the side opposite to the first gate electrode as a bottom electrodewith respect to the source electrode and the drain electrode,

characterized by comprising the steps of:

forming a sensor electrode of the photodetecting device and a conductivefilm for the gate electrodes of the thin film transistor onto thesubstrate;

forming the sensor electrode of the photodetecting device and the firstgate electrode of the thin film transistor by patterning the conductivefilm;

forming a common electrode of the photodetecting device and a conductivefilm for the source electrode and the drain electrode of the thin filmtransistor into an upper portion of the substrate;

forming the common electrode by patterning the conductive film;

forming the source electrode and the drain electrode of the thin filmtransistor by further patterning the conductive film;

forming a conductive film for the second gate electrode into a furtherupper portion of an insulating film on the conductive film; and

forming the second gate electrode by patterning the conductive film.

As described above, according to the invention, in the solid state imagepickup apparatus which comprises the photodetecting device and the TFTand in which the photodetecting device is arranged so as to overlap apart or whole surface of the TFT, by sandwiching the gap portion betweenthe source and drain electrodes of the TFT by the first and second gateelectrodes arranged vertically, the threshold voltage of the TFT is notchanged due to the external action of the photodetecting device arrangedin the upper portion and the stable TFT characteristics can be assured.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the claims.

1-10. (canceled)
 11. A solid state image pickup apparatus comprising: aphotodetecting device and one or more thin film transistors connected tosaid photodetecting device formed in one pixel, wherein a part of saidphotodetecting device is formed over at least a part of said thin filmtransistor, wherein said thin film transistor comprises a sourceelectrode, a drain electrode, a semiconductor layer, a first gateelectrode, and a second gate electrode arranged on the side opposite tosaid first gate electrode with respect to a channel portion of the asemiconductor layer, and wherein said first gate electrode and saidsecond gate electrode are connected to a common gate wiring.
 12. Thesolid state image pickup apparatus according to claim 11, wherein saidthin film transistor is a double gate type thin film transistorcomprising at least said first gate electrode, an insulating layer, saidsemiconductor layer, said semiconductor layer having an impurity doped,said source and drain electrodes, an insulating layer, and said secondgate electrode which are sequentially formed onto an insulatingsubstrate.
 13. The solid state image pickup apparatus according to claim11, wherein said second gate electrode covers at least a part of a gapportion between said source electrode and said drain electrode.
 14. Thesolid state image pickup apparatus according to claim 11, wherein eithersaid source electrode or said drain electrode is connected to a transferwiring connected to a signal processing circuit, and said second gateelectrode does not two-dimensionally overlap either said sourceelectrode or said drain electrode connected to the transfer wiring. 15.The solid state image pickup apparatus according to claim 11, whereinsaid second gate electrode and said first gate electrode are connectedto one gate driver circuit by a gate wiring and controlled by said gatedriver circuit.
 16. The solid state image pickup apparatus according toclaim 11, wherein said second gate electrode is formed as a filmsimultaneously with an electrode material constructing saidphotodetecting device.
 17. The solid state image pickup apparatusaccording to claim 11, wherein said photodetecting device is constructedby at least an insulating layer, a semiconductor layer, and asemiconductor layer having an impurity doped c.
 18. The solid stateimage pickup apparatus according to claim 11, wherein saidphotodetecting device is constructed by at least a first semiconductorlayer having an impurity doped, a semiconductor layer, and a secondsemiconductor layer having an impurity doped of a conductivity typeopposite to that of said first semiconductor layer having a impuritydoped.
 19. A radiation image pickup apparatus comprising thephotodetecting device of the solid state image pickup apparatusaccording to claim 11, as a radiation detecting device for directly andphotoelectrically converting a radiation.
 20. A radiation image pickupapparatus comprising the photodetecting device of the solid state imagepickup apparatus according to claim 11, and a wavelength convertarranged onto said photodetecting device.